Motion blur is a well-known problem in the imaging art that may occur during image capture using digital video or still-photo cameras. Motion blur is caused by camera motion, such as vibration, during the image capture process. It is somewhat rare to have a priori measurements of actual camera motion when motion blur occurs and as such, techniques have been developed to correct for motion blur in captured images.
For example, blind motion blur correction is a known technique for correcting motion blur in captured images. This technique corrects motion blur based on attributes intrinsic to the captured motion blurred image. Blind motion blur correction seeks to estimate camera motion parameters based on the captured image and then to employ the estimated camera motion parameters to at least partially reverse the effects of the motion blur in the captured image.
The process of estimating camera motion parameters is typically simplified by assuming camera motion has occurred linearly, and at a constant velocity. During camera motion parameter estimation, the linear direction of camera motion (i.e. the blur direction) is firstly estimated followed by the extent of the camera motion (i.e. the blur extent). The publication entitled “Comparison of direct blind deconvolution methods for motion-blurred images” authored by Yitzhaky et al. (Optical Society of America, 1999), discloses a method for estimating linear blur direction. During this linear blur direction estimation method, the direction in which a blurred image's resolution has been maximally decreased is found and is declared as the blur direction. This is achieved by high-pass filtering the blurred image in a number of directions and determining the direction, which yields the lowest intensity. This blur direction estimation technique unfortunately suffers disadvantages in that it is useful generally only in situations where sufficient edge information is available in each of the measured directions. For instance, this blur direction estimation technique may incorrectly declare the measured direction having the lowest intensity after high-pass filtering to be the blur direction in the situation where the low intensity is a result of scarce edge information in the original, unblurred image and not due to blur. Furthermore, this blur direction estimation technique is very sensitive to errors due to noise, which may in any particular direction tend to either sharpen or soften edges.
With the blur direction determined, the blur extent is then estimated typically using a correlation-based method thereby to complete the camera motion parameter estimation process.
Once the camera motion parameters (i.e. the blur direction and blur extent) have been estimated, blur correction is effected using the estimated camera motion parameters to reverse the effects of camera motion and thereby blur correct the image. The publication entitled “Iterative Methods for Image Deblurring” authored by Biemond et al. (Proceedings of the IEEE, Vol. 78, No. 5, May 1990), discloses an inverse filter technique to reverse the effects of camera motion and correct for blur in a captured image. During this technique, the inverse of a motion blur filter that is designed according to the estimated camera motion parameters is applied directly to the blurred image.
Unfortunately, the Biemond et al. blur correction technique suffers disadvantages. Convolving the blurred image with the inverse of the motion blur filter can lead to excessive noise magnification. Furthermore, with reference to the restoration equation disclosed by Biemond et al., the error contributing term having positive spikes at integer multiples of the blurring distance is amplified when convolved with high contrast structures such as edges in the blurred image, leading to undesirable ringing. Ringing is the appearance of haloes and/or rings near sharp objects in the image and is associated with the fact that de-blurring an image in an ill-conditioned inverse problem. The Biemond et al. publication reviews methods for reducing the ringing effect based on the local edge content of the image, so as to regulate the edgy regions less strongly and suppress noise amplification in regions that are sufficiently smooth. However, with this approach, ringing noise may still remain in local regions containing edges.
Various techniques that use an iterative approach to generate blur corrected images have also been proposed. Typically during these iterative techniques, a guess image is motion blurred using the estimated camera motion parameters and the guess image is updated based on the differences between the motion blurred guess image and the captured blurred image. This process is performed iteratively until the guess image is sufficiently blur corrected. Because the camera motion parameters are estimated, blur in the guess image is reduced during the iterative process as the error between the motion blurred guess image and the captured blurred image decreases to zero. The above iterative problem can be formulated as follows:l(x,y)=h(x,y)*O(x,y)+n(x,y)where:                l(x,y) is the captured motion blurred image;        h(x,y) is the motion blurring function;        O(x,y) is an unblurred image corresponding to the motion blurred image l(x,y);        n(x,y) is noise; and        A*B denotes the convolution of A and B.        
As will be appreciated from the above, the goal of image blur correction is to produce an estimate (restored) image O′(x,y) of the unblurred image, O(x,y), given only the captured blurred image, l(x,y). In the correction algorithm, h(x,y) is assumed to be known from the camera motion parameters. If noise is ignored, the error E(x,y) between the restored image, O′(x,y), and the unblurred image, O(x,y), can be defined as:E(x,y)=l(x,y)−h(x,y)*O′(x,y)
While iterative methods such as that described above provide some advantage over direct reversal of blur using motion blur filters, these known iterative methods are still often subject to overcorrection in some areas of the image, leading to non-uniform results and significant ringing. As will be appreciated improvements in blur correction are desired.
It is therefore an object of the present invention to provide a novel method and apparatus for reducing motion blur in an image and to a method and apparatus for estimating blur direction in a motion blurred image.